The present invention relates to a metal golf club head, and more specifically, to a configuration of a face portion that improves the initial speed of the hit ball.
The present inventors have found from the invention described in Japanese Patent Laying-Open No. 2-167181 that the initial speed of the hit ball increases when a golf club head (hereinafter referred to as a head) is designed to have a natural frequency close to that of the ball. Here, the natural frequency of the head is meant to refer to the natural frequency of the face portion at the time it undergoes the impact of a hit. There are various methods of making the head natural frequency smaller than is conventional to design it to be close to the ball natural frequency. The most effective method is to lower the rigidity of the face portion. Generally, as a method of lowering the rigidity of the face portion, producing a thinner face thickness or using a low elasticity modulus material may be considered.
On the other hand, as a model of an iron golf dub, the model having a face member made of titanium alloy fit into the head body is recently becoming popular. The main aim of this model is to achieve an enlarged sweet spot area by utilizing titanium having a light specific gravity for the face portion so that the head periphery portion becomes heavier in weight. Conversely, some models employ a material having a large specific gravity such as tungsten for the sole portion in order to lower the center of gravity of the head.
Moreover, still another model of a metal iron golf club formed by a head body and a face member made of different materials has an opening that penetrates from the face to the opposite side of the face and has the face member fit into the opening. Thus, the face member is exposed inside the cavity. Titanium alloy is mainly employed as the material for the face member. The effect achieved by this model is similar to that achieved by the model in which a plate material made of titanium alloy is fit in as the face member as described earlier. The main objective here, again, is the further enlargement of the sweet spot area achieved by exposing the plate material of titanium alloy inside the cavity. Thus, the plate material fit into the opening is generally smooth except for the scoring lines. The plate material itself in the portion exposed inside the cavity does not show much unevenness besides the logo impression and the like designating the name of the club and the material used.
Since the properties of titanium alloy involves a light specific gravity as well as a low elasticity modulus, titanium alloy shows promise in that it is a material that can lower the rigidity of the face portion, which is a requirement for lowering the head natural frequency. The head utilizing titanium alloy for the face member, however, generally has a cavity structure so that the face thickness of the surface for hitting the ball, though thinner than that of a plain-back type iron golf club, is considerably thick in comparison with the face surface of a wood head. In addition, because the iron golf club and the wood golf club have different structures, their natural frequencies in the range of approximately 1800 Hz to approximately 2000 Hz are largely different from the natural frequencies of the balls in the range of about 800 Hz to about 1000 Hz that vary depending on the type of the ball.
On the other hand, by making the face thickness evenly thin, the strength of the face portion is reduced, leading to the problem of destruction by a hit ball. Thus, as described in Japanese Patent Laying-Open No. 9-192273, there is an invention of a structure in which the face thickness around the sweet spot area has a sufficient strength against the impact due to collision between the ball and the face portion while the face thickness in the region other than the sweet spot area is made thin.
In the head described in Japanese Patent Laying-Open No. 9-192273, however, if the strength of the sweet spot portion on the face is ensured while the portion surrounding the sweet spot portion is made thin, the spring performance of the face portion may certainly improve, although at the same time, the impact of the ball hitting the sweet spot would be supported by the surrounding portion. Since the structure is not designed to support such an impact, the sweet spot portion of the face portion may possibly disadvantageously cave in in the case of an average male golf player hitting the ball with the head speed of about 40 m/s.
Therefore, the main object of the present invention is to clarify the relation between the rigidity and the natural frequency of the face portion and to provide a metal golf club head having the smallest possible head natural frequency while ensuring the strength of the face portion.
The present invention is a golf club head formed of a metal material and having a medium sensitivity region and a low sensitivity region provided on the face portion for hitting a ball, where the face thickness of a part or all of the medium sensitivity region is made thinner than the face thickness in the vicinity of the face center that exist within the low sensitivity region.
According to another aspect, the present invention is a metal golf club head formed by a head body and a face member formed of different materials and having an opening that penetrates from the face to the opposite side of the face with the face member fit into the opening, where a medium sensitivity region and a low sensitivity region are provided on the face portion for hitting the ball, with the face thickness of a part or all of the medium sensitivity region made thinner than the face thickness of the vicinity of the face center that exists within the low sensitivity region.
According to a still another aspect of the present invention, the face thickness of the face portion is varied so that the face thickness of a portion which is within the medium sensitivity region and on the sole side from the low sensitivity region and which is a part or all of the medium sensitivity region excluding the region extending on either side of the face centerline that is within the range of a predefined length of the entire length of the face portion in the toe-heel direction is thinner than the face thickness in the vicinity of the face center existing within the low sensitivity region.
According to a further aspect of the present invention, a region within the medium sensitivity region and having its face thickness made thin is defined as the low rigidity region, and the area of the low rigidity region is selected to be 5% to 50% of the face area.
According to a still further aspect of the present invention, the region other than the low rigidity region in the face member is defined as the high rigidity region, and the face thickness of the low rigidity region is formed at least 0.2 mm thinner than the thickest face thickness of the high rigidity region.
According to a still further aspect of the present invention, the face thickness of the low rigidity region is formed at least 0.5 mm thinner than the thickest face thickness of the high rigidity region.
Further, the face thickness of the low rigidity region is selected to be 0.5 to 2.7 mm. In addition, according to a still further aspect of the present invention, a material having a tensile strength of 100 kgf/mm2 or greater and having a Young""s modulus of elasticity of 10000 kfg/mm2 or lower is used for the face member.
According to a still further aspect of the present invention, a recessed portion is formed on the face portion from the face of the head toward the opposite side of the face, and the face member is fit into the recessed portion.
According to a still further aspect, the present invention is a metal golf club head having a hollow portion and formed such that the rigidity of a portion on the heel side of the face portion is made lower than that on the toe side. The head is formed such that the rigidity of the portion excluding the vicinity of the face center is lowered, where the region with the lowered rigidity is defined as the low rigidity region, and the area of the low rigidity region is selected to be in the range of 5% to 50% of the face area.
Moreover, the low rigidity region is formed above the face center or in the lower half region on the heel side.
According to a still further aspect of the present invention, when the region other than the low rigidity region of the face portion is defined as the high rigidity region, the face thickness of the low rigidity region is selected to be thinner than the face thickness of the high rigidity region. Moreover, the face thickness of the low rigidity region is formed at least 0.2 mm thinner than the face thickness of the high rigidity region, and more preferably, a material having a lower elasticity modulus than that of the material used in the high rigidity region is used for the face portion in the low rigidity region.